In a typical automobile gas turbine engine, ambient inlet air is supplied by a compressor or gas generator at comparatively low temperatures and moderate pressures and preheated by flowing through an exhaust heated regenerator. The preheated inlet air is then conducted to a combustion chamber or burner where fuel is added and burned. The hot gases or combustion products from the burner are directed to the gas turbine rotor stages to drive the latter and power the compressor as well as the driving wheels of the automobile. The exhaust gases from the rotor stages contain appreciable heat energy which is transferred to the inlet air from the compressor via the aforesaid regenerator. The resulting appreciably cooled exhaust gases are then discharged to the atmosphere.
Without some provision to the contrary, the comparatively high combustion temperature in the burner creates an objectionable quantity of nitrogen oxides referred to hereinafter as NOx. Various burner designs and modes of operation have been proposed to minimize NOx formation during the combustion process. Such designs may be classified according to whether the geometry of the burner is variable or fixed. Burner systems having means for varying their size and/or shape in accordance with the operating mode of the engine have been fairly effective in reducing NOx formation, but such burners have required costly and sophisticated controls for the burner geometry.
The present invention is directed to a fixed geometry burner design wherein liquid hydrocarbon fuel is supplied to premixing chambers in the nature of a fog of finely dispersed droplets mixed with air and then vaporized. No external heat is added to the fuel prior to its entry into the premixer except incidentally from the environment of the hot engine. Heated air is supplied in controlled amounts to the premixer and the dispersed fuel droplets therein are vaporized and thoroughly mixed with the air to provide a lean combustible mixture. Several successive premixer stages may be employed and the mixture from each stage is ignited and burned for a controlled time period, whereupon the combustion temperature is rapidly reduced by the addition of cooler air or a lean fuel-air mixture from the next successive stage. The rate of combustion and the resulting temperature are predetermined for each stage by predetermining the fuel to air ratio in the mixture for that stage.
A number of suitable fuel dispersing nozzles are presently available to produce the desired fuel-air dispersion, thereby to expedite fuel vaporization and enhance engine operation. Also although the present invention is concerned primarily with liquid hydrocarbon fuel, the burner system described herein can also be employed with other liquid fuels, such as alcohol by way of example, or gaseous fuels.
It has been found in accordance with the present invention that if the fuel and a predetermined fraction of the compressed air are premixed and the fuel is substantially vaporized prior to combustion to provide a homogenous lean charge, the subsequent burning will provide low levels of contaminants, such as NOx in particular as well as unburned hydrocarbon (HC) and carbon monoxide (CO). However, in practice the gas turbine engine must operate over such a large fuel-air range that the desired low levels of the contaminants is not readily obtained.
It is an important object of the present invention to provide an improved fixed geometry burner or combustion system for an automobile gas turbine engine that achieves significant advantages of the variable geometry burner without their complexity and expense and which eliminates the necessity for sophisticated and costly control systems with their inherent problems of reliability, servicing, and associated problems.
Other and more specific objects are to provide both an improved combustion system of the above character and a method of operating a gas turbine engine utilizing the system, wherein lean supplies of fuel and air are thoroughly premixed at predetermined elevated temperatures in a number of premixing stages and thereafter substantially completely burned at controlled temperatures and in a limited time period at various locations along the flow path of the combustion gases. Each premixing stage either by itself or in combination with one or more of the other premixing stages supplies the fuel required for a predetermined range of steady state engine operating conditions. The first stage fuel-air mixture is preferably ignited at an upstream location in the combustion flow path and the resulting hot combustion products are employed to ignite the lean fuel-air mixtures of any subsequent stages.
The formation of NOx increases as either the temperature or time duration of the combustion process increases. Accordingly these factors are reduced as much as feasible. Combustion temperature decreases as the fuel-air ratio is reduced from the stoichiometric value, but the difficulty of igniting the fuel-air mixture and maintaining combustion increases with consequent increased CO and unburned HC in the combustion products. By increasing the precombustion temperature of the fuel-air mixture, ignition and combustion of leaner mixtures is enhanced, but of course the resulting combustion temperature is then increased. All of the above factors are taken into consideration.
In accordance with the present invention, fuel-air mixtures provided in the various stages are preferably near the lean limit that will support combustion when the engine is operating at the minimum fuel requirements for that stage, and the combustion supporting inlet air is supplied to the mixture at near the maximum temperature of the preheated air from the regenerator. Although three or more stages are within the scope of the present invention, it is desirable for the sake of structural simplicity and economy to utilize as few stages as feasible, depending on the size and character of the engine. An important object is to provide a burner of the above character wherein the first stage is dimensioned to operate over as large a fuel range as possible beyond the minimum fuel requirement for the engine.
A criterion limiting the maximum dimensions for the first stage premixer is that the latter's fuel-air mixture must readily ignite and burn substantially completely when the engine is operating at its lowest fuel requirement. As the fuel to the first stage premixer increases, the difficulty of ignition and complete combustion at the lean mixtures involved diminishes. On the other hand it will be apparent from the description herein that as the first stage apparatus is increased in size to operate satisfactorily with increasing amounts of fuel, a size will be reached where the minimum fuel requirement for the engine will not be sufficient to permit ignition and combustion.
The first and second stage apparatus and fuel-air mixtures are also predetermined so that the resulting first stage combustion temperature will be sufficient to ignite the fuel-air mixture from the second stage when the latter mixture is in its nominal lower range, i.e. during idle operation of the engine. It has been found that if the first stage fuel to air ratio is between approximately one-third and one-half the stoichiometric value, i.e., between approximately 0.023 and 0.035 by weight for hydrocarbon fuels where the stoichiometric value is approximately 0.067, combustion on the order of 90% or more complete is believed to be obtained, and at any rate the combustion temperature is sufficiently low and for a sufficiently short time interval that excessive NOx formation is avoided. (Note that all fuel-air ratios herein are by weight).
The first stage fuel-air mixture is ignited and burned in a first stage reactor dimensioned to enable approximately 90% complete combustion in the required short time interval and limited temperature. The temperature of the first stage combustion products is then reduced rapidly by quenching with an appreciably cooler second stage air stream or a lean premixed second stage fuel-air mixture, thereby to retard continued NOx formation.
In one embodiment of the invention, the first stage fuel operates the engine at its idle condition. At that condition, the second stage premixer supplies only quench air to cool the hot first stage combustion products as soon as combustion is substantially complete as aforesaid. When the engine load increases from the idle condition, fuel to one or both stages is increased and thoroughly mixed with the air for the corresponding stage. The fuel in the second stage mixture ignites as it comingles with the hot first stage combustion products. The mass of second stage air is approximately twice that of the first stage air, so that at or near the idle operating condition when no second stage fuel is supplied, the temperature of the resulting first and second stage mixtures may be as low as approximately 1800.degree. F., well below the temperature of rapid NOx formation yet hot enough to continue HC and CO reactions. As the engine load and second stage fuel increase from the idle condition, the second stage fuel air ratio gradually increases but is not allowed to exceed approximately one-half the stoichiometric ratio during ordinary steady state operation of the engine, as for example up to approximately 80% of maximum engine or compressor speed, or approximately 75 to 80 mph for the specific engine involved, comprising a 150 horsepower engine driving approximately a 4300 lb. vehicle. Thus, the temperature of the comingled first and second stage combustion products is maintained below the level of rapid NOx formation, as for example below approximately 3000.degree. F. (Note that all reference to operating conditions herein apply to steady state conditions, rather than to acceleration or deceleration conditions, unless specifically stated otherwise).
Similarly to the first stage reactor, the second stage reactor in which the second stage fuel-air mixture is burned is dimensioned so that substantially complete combustion is obtained in a sufficiently short time interval that NOx formation is nominal. At the end of the latter time interval, the second stage combustion temperature is reduced rapidly by the addition of comparatively cool third stage quench air amounting to approximately four times the mass of the second stage air, thereby to cool the resulting mixture during normal steady state operation of the engine to between approximately 1300.degree. F. (at idle operation) and 1800.degree.-1900.degree. F. at high speed operation. NOx formation is thus stopped almost completely as the resulting mixture is conducted to the turbine rotor stages. Likewise HC and CO in the combustion products are insignificant by the time of the second quench.
Another object is to provide such a gas turbine combustion system having two fuel supply stages. The first stage is dimensioned to supply the curb idle power requirements for the engine and comprises a comparatively small first stage premixer that receives about 10% of the engine air and an amount of fuel to achieve a lean fuel-air ratio less than approximately one-half the stoichiometric value. The first stage premixer may comprise a conical chamber or extension of a fuel and air dispersing nozzle or fuel atomizer of conventional design for emitting a fog or fine dispersion of fuel droplets and air at high velocity coaxially into the small end of the conical first stage premixer. The amount of air, if any, required by the nozzle for dispersion of the fuel is comparatively small with respect to that required for the first stage premixer, so supplemental preheated air is injected through the conical sidewalls of the first stage premixing chamber to enhance turbulence and mixing of the fuel and air therein and to assure substantially complete evaporization of the fuel prior to ignition.
The large end of the conical first stage premixer discharges its thoroughly mixed fuel and air into one end of a comparatively small coaxial tubular first stage reactor, where additional air may be added and turbulent mixing is effected upstream of an electrical igniter. The igniter located in the first stage reactor ignites the mixture which burns as it progresses along the tubular reactor until the combustion is at least 90% and usually more than approximately 98% complete. The hot burning gases are then discharged from the first stage reactor into a second stage reactor, which in a preferred embodiment comprises a main burner, at temperature amounting to between approximately 2700.degree. F. and 3200.degree. F.
The second fuel stage of the burner system comprises a conical second stage premixer appreciably larger than the first to supply a large portion of the engine power requirements in excess of that required for idle operation. Similarly to the first stage premixer, fuel is supplied as a fog or finely dispersed mixture of fuel droplets and air into the small axial end of the second stage premixer via a fuel dispersing nozzle or atomizer. The fuel atomizers for two stages may or may not be of the same type and either may or may not use air to disperse the fuel.
Supplemental heated inlet air is injected through the conical sidewalls of the second stage premixer to evaporate the fuel and create a turbulent thorough mixing of the fuel and air within the second stage premixer prior to discharge of the second stage fuel-air mixture into the main or second stage burner. The total air supplied to the second stage premixer will amount to approximately 18% of the total air from the engine and will effect a second stage fuel to air ratio less than approximately one-half the stoichiometric value. The second stage premixer does not employ an igniter but the fuel-air mixture discharged therefrom is ignited by the hot combustion products from the first stage as the first and second stage gases comingle within the main burner.
By virtue of the foregoing, shortly after the initial ignition the engine obtains its operating temperature. The engine heat thus derived from the combustion system and recovered from the exhaust gases from the rotor stages via regeneration is thus available almost immediately to preheat the fuel-air mixtures within the premixing stages and to assist in vaporizing the fuel. On the other hand, the engine is not dependent on the preheating and vaporization for operation. The engine will readily start in a cold condition by igniting a diffusion of fuel droplets and air discharged from the premixing stages.
The first and second stage fuel atomizers may employ comparatively cool inlet air directly from the gas turbine compressor, or may employ air preheated by the regenerator. Also, either fuel atomizer may be of the air blast nozzle type which employs comparatively large quantities of air at high velocity and low pressure to disperse the fuel, or may be of the air atomizing nozzle type which employs an auxiliary air-pump to supply smaller quantities of the atomizing air at appreciably higher pressure to disperse the fuel, or may be effective without the use of air to disperse or "atomize" the fuel.
Other objects of this invention are to provide an improved combustion system for a gas turbine engine that appreciably reduces undesirable exhaust emissions of HC, CO and NOx during acceleration of the engine; and in particular to provide such a system wherein fuel-air ratios appreciably richer than stoichiometric are supplied to the successive combustion stages during engine acceleration, such that substantially all the available oxygen is consumed, the resulting combustion temperature is considerably below the corresponding temperature for stoichiometric mixtures, and NOx formation is thus substantially avoided. Adjacent the downstream end of the final combustion stage and appreciably upstream of the turbine rotor stages, a large excess of comparatively cool air is added to the hot combustion products (which are comparatively rich in unburned HC and CO) to cool the same below the temperature at which NOx formation is excessive and also to provide adequate air to complete the oxidation of Co and unburned HC and effect a resulting temperature approximating 2700.degree. F. by the time these combustion products are directed into the turbine rotor stages.
Other objects of this invention will appear in the following description and appended claims, reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.